LONDON — A little over nine months after British voters chose to withdraw from the European Union, Britain took a decisive — and likely irreversible — step Wednesday toward leaving a partnership that has bound the country to the continent for nearly half a century.
With the simple handoff of a letter in Brussels in the early afternoon, the British government became the first to trigger Article 50 — the mechanism for nations to exit the European Union.
“This is a historic moment from which there can be no turning back,” Prime Minister Theresa May announced to a momentarily hushed House of Commons, before debate later turned rowdy.
In Brussels, European Council President Donald Tusk said there was “no reason to pretend that this is a happy day.”
“After all,” a visibly upset Tusk said, “most Europeans, including nearly half the British voters, wish that we would stay together, not drift apart.”
The move instantly plunged both Britain and the 27 other E.U. nations into two years of what will almost certainly be messy and acrimonious negotiations over the terms of divorce.
The talks will encompass a dizzying array of subjects, including trade terms, immigration rules, financial regulations and, of course, money. Britain joined the group that became the European Union in 1973, so decades of ties, pacts and arrangements are part of the complex unraveling.
For both sides, the stakes are enormous.
Britain could be forced to reorient its economy — the world’s fifth largest — if it loses favorable terms with its biggest trade partner. It also may not survive the departure in one piece, with Scotland threatening to bolt.
The European Union, which for decades has only expanded its integrative reach, faces perhaps an even greater existential threat. If Britain is allowed to get a good deal, other countries contemplating their own departures could speed toward the exits.
The formal declaration of Britain’s intention came in the form of a six-page letter from May to Tusk. The letter, which opened with the handwritten salutation “Dear President Tusk,” was delivered by Britain’s ambassador to the E.U., Tim Barrow.
Tusk later tweeted a photo of the moment he received the letter as the men stood in front of E.U. flags and Union Jacks. Barrow appeared to be grinning; Tusk was grimacing.
From either side of the English Channel on Wednesday, there were attempts to take the heat out of what had become a grievance-filled split even before it officially got underway.
The top diplomat for the European Union’s most powerful member, Germany, said he wished Britain well.
“The stale-sounding sentence used in private life after a divorce, ‘Let’s remain friends,’ is right in this case,” said German Foreign Minister Sigmar Gabriel.
May’s letter, meanwhile, ratcheted down earlier threats to walk away from talks and leave with no deal — an option popularly known as “dirty Brexit” — if the E.U. offers are not to her liking.
The letter urged the European Union to let Britain go “in a fair and orderly manner, and with as little disruption as possible on each side.”
May has said that Britain will prioritize regaining control over immigration and removing Britain from the jurisdiction of the European Court of Justice. She has also acknowledged that Britain will not remain a member of Europe’s common market or its customs union. Instead, she has sought a new trade deal that reflects, as the letter described it, Britain’s “deep and special partnership” with the European Union.
Despite an overall tone that is more conciliatory than previous statements, the letter also unleashed some implicit threats. It raised, for instance, the specter that Britain could reduce its contributions to European intelligence and security if London does not get what it wants in a trade deal.
“In security terms a failure to reach agreement would mean our cooperation in the fight against crime and terrorism would be weakened,” she wrote.
Although some legal experts say that an Article 50 declaration is reversible, British and E.U. officials have both said they believe it is not.
The British public stunned the world last June when it opted to leave, voting 52 percent to 48 percent in a referendum. Polls show that voters who opted for “leave” were driven by concerns that immigration was out of control under the E.U.’s free-movement laws, and that Britain needed to leave the bloc to restore its sovereignty.
Advocates for “remain” had forecast grievous economic harm and a weaker British role in global affairs.
As Britain prepares to leave, it continues to be deeply divided. Opinion polls show that the country is split almost as evenly today as it was last June.
The still-raw divisions were on vivid display Wednesday when May made her case to members of Parliament. She was cheered by Brexit backers and jeered by its opponents as she announced that Britons “are going to make our own decisions and our own laws. We are going to take control of the things that matter most to us.”
After May ticked off the potential benefits of Brexit, the opposition leader, Labour Party head Jeremy Corbyn, enumerated the possible pitfalls, calling the prime minister’s Brexit strategy “reckless and damaging.”
The triggering of Article 50 was a victory for May, who stepped into the vacuum left last summer when her predecessor, David Cameron, abruptly resigned after the public disregarded his call for the country to stay in the European Union.
Although May was herself quietly in favor of “remain” during the campaign, she pivoted quickly in the aftermath of the vote and adamantly maintained that she would make good on the public will. “Brexit means Brexit,” she repeatedly declared.
It was not until January, however, that May gave true shape to what Brexit might mean. In a speech at London’s Lancaster House, May made the case for a clean break from the European Union, saying she did not want a deal that would leave Britain “half-in, half-out.”
But May’s pitch has done little to bring the country together on Brexit.
Of the four nations that make up the United Kingdom, only two — England and Wales — voted for Brexit. The other two, Scotland and Northern Ireland, came down against it.
Scotland’s semiautonomous Parliament voted on Tuesday to seek another independence referendum. Advocates argue that an E.U. departure against the will of Scottish voters has sufficiently changed the calculus since the last independence vote, in 2014, that a new one is justified.
Irish nationalists in Northern Ireland have also used Brexit to renew their decades-long efforts to break away from Britain.
Amid British divisions, Europe has taken an unusually united stand in asserting that Britain will not be able to secure a better deal than the one it has today. If it does, German Chancellor Angela Merkel and other stalwart defenders of the E.U. fear that Britain could be just the start of a broader splintering.
Tusk, a former Polish prime minister, said the remaining 27 E.U. nations would hold firm in negotiations over the coming two years. A first statement of bargaining positions is expected Friday.
“Our goal is clear,” Tusk said. “To minimize the costs for the E.U. citizens, businesses and member states.”
Because of French elections this spring, along with German elections in the fall, Britain’s E.U. divorce talks are likely to get off to a slow start. Once the negotiations begin in earnest, there will be little time to finish. The talks are capped at two years, meaning they must be complete by March 2019.
Despite the risks, Britain’s impending exit was celebrated Wednesday by the country’s staunchly pro-Brexit tabloids.
“Freedom!” exulted the front page of the Daily Mail.
The mood was more sober across the English Channel. Before walking away from the podium after delivering his remarks, Tusk had a poignant final message for Britain:
“We already miss you.”